The present invention relates to magnetic domain memory devices and more particularly to an improved drive coil assembly for use in such devices.
Magnetic domains or bubbles are minute cylindrical areas that can be generated and maintained in thin films or chips of magnetic material. The thin films can be used to store binary data. The presence of a domain can represent a binary 1. The absence of a domain can represent a binary 0. The domains, which are magnetized oppositely from the rest of the thin film material, can be propagated along Permalloy tracks on the film surface by a rotating magnetic field generally referred to as a drive field. The drive field is generated by an assembly consisting of independently-energizable orthogonal coils, normally referred to as X and Y drive coils. The lines of force in a drive field are generally parallel to the surface of the domain material chip. Track arrangements for performing shift functions and logic operations are well known in the art.
The strength of the magnetic drive field acting on a domain is critical. If the drive field is too strong, a domain may be forced from a track under certain conditions, with resulting loss of a data bit. A drive field which is too strong may also cause a domain to be propagated farther than intended, particularly at corners of the track. The data is naturally distorted by such an occurrence.
Conversely, a drive field which is too weak may fail to propagate a domain as intended. Such a failure also results in distortions in the stored data.
Current practice calls for several thin films or chips to be mounted on a single ceramic substrate and for several substrates to be stacked in single memory device. The drive field acting on the domains in any one of these chips must always be strong enough to propagate the domains but yet not so strong as to force the domains from the tracks or farther along the track than intended.
In prior art drive coils, the conductors in each coil are uniformly spaced. The magnetic field generated by such a coil has a peak distribution which has a maximum at the center of the memory device, but which falls toward a minimum value as the edges of the coil are approached. Placing all chips only in areas known to be subject to magnetic fields having acceptable minimum and maximum values assures that magnetic domains can be consistently propagated. However, the number of chips which can be placed is necessarily limited as is the data storage capacity of such an assembly.
The data storage capacity of a magnetic domain memory device can be increased by increasing the area suitable for chip placement. In the prior art, attempts have been made to increase the useful area by the simple expediency of increasing the overall physical size of the drive coil assembly. There are drawbacks to this approach. A larger drive coil presents a greater impedance during high frequency operations. As a result, increased amounts of power must be dissipated. Moreover, the rate at which bubbles can be propagated across the thin film surface is limited by the frequency-dependent inductive component of the coil impedance.